Sliding sync pollers should use set_presence=offline when requesting oldschool /sync

[reivilibre]

/sync currently bumps the user's presence timeout. Since SS pollers are active even when the device is offline, this is undesired as the users will be marked as online when they are not. Further, this causes a bit more load on Synapse and also as a consequence causes other sync loop within Synapse to wake up to emit this unwanted presence data to other users who may be syncing.

I suspect this may be contributing to the high CPU usage reported at matrix-org/synapse#15980.

(or, with extra involved effort: find some way to pass the right state through from the client)

Related code:

sliding-sync/sync2/client.go

Line 64 in aadc358

func (v *HTTPClient) DoSyncV2(ctx context.Context, accessToken, since string, isFirst, toDeviceOnly bool) (*SyncResponse, int, error) {

→

sliding-sync/sync2/client.go

Line 88 in aadc358

func (v *HTTPClient) createSyncURL(since string, isFirst, toDeviceOnly bool) string {

[DMRobertson]

What we really want is a way to express "don't alter presence based on this /sync call". It seems like set_presence=offline does this for Synapse, but not for Dendrite. The spec is ambiguous. See matrix-org/matrix-spec#1609.

I don't have any strong opinions about what the right thing to do is.